A conventional commercial use of a particulate sensor is a smoke detector, and an example description for such a sensor can be found in Nishikawa et al., “Cyber Sensor,” Matsushita Electric Works Technical Report, Matsushita Electric Works, Ltd., February 2003, No. 80, pp. 75-83 (“Report”). In the Report, a smoke detector senses smoke by utilizing scattered infrared light, where an emitter and a detector are installed in a chamber surrounded with a reflecting plate (called a labyrinth). The labyrinth has a structure in which smoke can flow in (from outside) so that smoke can block the infrared from reaching the detector. In other words, the smoke is sensed by the change in the quantity of light received by detector.
In order to determine the concentration of smoke based on a slight quantity of received light, it is generally necessary to amplify analog signals that are output from the detector using a low-noise and high-gain amplifier. Additionally, an analog-to-digital converter (ADC) is also used to enable further digital processing. Therefore, in the conventional smoke sensor, the scale of the circuit is enlarged due to the high-gain amplifier the ADC, and there is a need for smaller circuits.